Up to now, steel plants have been producing steel sheets of various thicknesses and dimensions by the hot rolling process. In the hot rolling process, a thin steel sheet called a coil is produced from a thick steel plate called a slab by rolling out the slab held from above and below by a set of work rolls of a rolling mill. The surface quality of a coil thus produced depends on the surface condition of the work rolls. In addition, rolling a plurality of thick plates one after another using a certain set of work rolls gradually deteriorates the surface condition of the work rolls. For this reason, the rolling of a steel plate requiring a high quality is preferably done with a set of work rolls while the rolls are still fresh.
Moreover, rolling a steel plate may sometimes leave a groove, as wide as the plate, on the surface of the work rolls. Accordingly, the groove left on the surface of the work rolls by a narrower steel plate rolled earlier may sometimes leave a flaw on the surface of a wider steel plate that is rolled later. Furthermore, the specification of rolling equipment or the like limits, within a certain range, the difference in thickness, if any, between two steel plates that are successively rolled. Still furthermore, especially thinner steel plates cannot be successively rolled in large numbers, in order to prevent the deterioration in the durability of work rolls. As seen from above, various constraints on the sequence of rolling steel plates should be satisfied to maintain the quality of steel sheets and to improve the productivity.
Studies have been done thus far on a problem for determining, by computation, a sequence of objects under certain constraints (refer to Japanese Patent Application Publications No. 2000-167610 and No. 2004-209495). The kind of problem is called a scheduling problem. The framework of the conventional scheduling problem, however, has difficulty in dealing with a scheduling problem in the hot rolling process, due to the large scale of the problem and various constraints, such as those mentioned above, imposed on the process. To solve a large scale problem within a practical time, a conventionally known method is, firstly, finding a feasible solution, which is not optimum but satisfies the constraints, and then gradually improving the feasible solution. In the case of the hot rolling process, however, just to find a feasible solution is sometimes difficult. To be more precise, it is sometimes difficult just to find a solution that, even though not being an optimum solution, satisfies many constraints of different characteristics. The constraints include, for example, a local constraint such as the defining of the relationship between steel plates to be successively rolled. The constraints also include a global constraint, such as the sequencing of the rolling operations of the steel plate as a function of the durability of work rolls.